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JAVASCRIPT

Chapter 10: “Error Handling & Debugging”

Order of execution

  • scripts are processed in order in which statements are executed, and some tasks cannot complete until another statement or function has been run.

Execution contexts

  • GLOBAL CONTEXT:code that is in the script, but not in a function.

There is only one global context in any page.

  • FUNCTION CONTEXT:code that is being run within a function.

Each function has its own function context.

VARIABLE SCOPE

  • GLOBAL SCOPE:when declared a variable outside a function, it can be used anywhere because it has global scope.
  • FUNCTION-LEVEL SCOPE:when a variable is declared within a function,it can only be used within that function.

The stack

  • Javascript executed on line of code at a time,and when a statement need a data from another line it will be stack that line before the cureent task.

EXECUTION CONTEXT & HOISTING

  • When enters a new execution context, there are two phases of activity:
    1. prepare:the new scope is created,and the variables, functions, and arguments are created,the value of the this keyword is determined.
    2. execute:now it can assign values to variables,reference functions and run their code,execute statements.

Errors

  • ERROR OBJECTS:error objects can help you find where your mistakes are and browsers have tools to help you read them.

It will be appear in the JavaScript console/Error console of the browser.

  • TYPES OF ERRORS:

    1. Syntax Error:SYNTAX IS NOT CORRECT,this is caused by incorrect use of the rules of the language. It is often the result of a simple typo.
    2. ReferenceError:VARIABLE DOES NOT EXIST,this is caused by a variable that is not declared or is out of scope.
    3. Type Error:VALUE IS UNEXPECTED DATA TYPE,this is often caused by trying to use an object or method that does not exist.
    4. RangeError:NUMBER OUTSIDE OF RANGE,if you call a function using numbers outside of its accepted range.
    5. Error:GENERIC ERROR OBJECT,the generic Error object is the template or prototype from which all other error objects are created.
    6. NaN:NOT AN ERROR,if you perform a mathematical operation using a value that is not a number, you end up with the value of NaN, not a type error.

  • HOW TO DEAL WITH ERRORS

    1. Debug the scriptt to fix the errors:to track down the source of the error and fix it.
    2. Handle errors gracefully:you can handle errors gracefully using try, catch,throw, and finally statements.

  • A DEBUGGING WORKFLOW TECHNIQUES

    • Where is the problem?
    • What exactly is the problem?

The JavaScript console will tell you when there is a problem with a script,where to look for the problem, and what kind of issue it seems to be.

In the internet console, the console will show you when there is an error in your JavaScript. It also displays the line where it became a problem for the interpreter.

  • DEBUGGING TIPS
    • Try another browser,some problems are browser specific.
    • Write numbers to the console so you can see which the items get logged.
    • Remove parts of code, and strip it down to the minimum you need.
    • Programmers often report finding a solution to a problem while explaining the code to someone else.
    • SEARCH:stack Overflow is a Q+A site for programmers.
    • Use the CODE PLAYGROUNDS
    • Use the VALIDATION TOOLS that can help you try to find errors in your code.

For JAVASCRIPT http://www.jslint.com,,,,http://www.jshint .com

  • COMMON ERRORS
  • this is a list of common errors you might find with your scripts.
    1. GO BACK TO BASICS:javaScript is case sensitive so check your capitalization.
    2. MISSED/ EXTRA CHARACTERS:every statement should end in a semicolon.
    3. DATA TYPE ISSUES:using= rather than == will assign a value to a variable, not check that the values match.

<An over all about Error Handling & Debugging>

  • If you understand execution contexts which have two stages and stacks, you are more likely to find the error in your code.
  • Debugging is the process of finding errors. It involves a process of deduction.
  • The console helps narrow down the area in which the error is located, so you can try to find the exact error.
  • JavaScript has 7 different types of errors. Each creates its own error object, which can tell you its line number and gives a description of the error.
  • If you know that you may get an error, you can handle it gracefully using the try, catch, finally statements.
  • Use them to give your users helpful feedback.

@Jon Duckett/JAVASCRIPT